The new slow-cookers all cook much hotter than the old ones, and food is cooked much faster. Too fast. I have had three of the old style, and three of the new style, and they all cook just as I have described. I even went to a car-boot sale to get one of the old ones, I was so annoyed that I couldn't leave my porridge cooking overnight, really gently and smoothly. If I had left porridge on overnight in one of the new ones, I'd have had a solid clod, or it might even have burst into flames. These new ones cannot be called slow cookers, but just electric pots. Not even stewpots. I can cook stews more gently in an ordinary oven than in a new "slowpot".
DO take it back and complain. I wish I had, but I hadn't seen other complaints like yours.